nbd: add pre_defined_connections module parameter for pre-created devices

blk_mq_update_nr_hw_queues() in nbd_start_device() may cause a queue
freeze.  The previous commit addressed this for newly created nbd
devices by setting the expected nr_hw_queues in nbd_dev_add().  However,
when reusing an old inactive nbd device, the queue freeze can still
occur if the old nbd->tag_set->nr_hw_queues does not match the new
socket connection count.  Inactive nbd devices can originate from two
sources: loading the nbd module with nbds_max, which sets the default
nr_hw_queues to 1, and the netlink method, which sets nr_hw_queues
according to the expected number of socket connections.  For the first
case, add a module parameter so the default nr_hw_queues can be
changed.  Users who know their expected number of connections can then
prevent queue freezes on pre-created devices via nbds_max.

Before this patchset:
real    0m2.195s
user    0m0.005s
sys     0m0.022s

After this patchset:
real    0m0.090s
user    0m0.004s
sys     0m0.018s

Signed-off-by: Yang Erkun <yangerkun@huawei.com>
Reviewed-by: Yu Kuai <yukuai@fygo.io>
Link: https://patch.msgid.link/20260805122930.57647-9-yangerkun@huawei.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

0 files changed, 0 insertions, 0 deletions